The primary goals of this project are the use to determine fundamental molecular mechanisms of action and toxicity of ligands of the aromatic hydrocarbon receptor (AHR) and the application of this mechanistic information to address specific needs in the assessment of human health risk posed by exposure to these compounds. There are a wide variety of AHR ligands including persistent lipophilic polyhalogenated aromatic hydrocarbons (e.g. 2,3,7 8-tetrachlorodibenzo-p-dioxin (TCDD), polychlorinated-dibenzodioxins, -dibenzofurans and -biphenyls), non-persistent polycyclic aromatic hydrocarbons (e.g. 3-methylcholanthrene) and endogenous/dietary ligands (e.g. indole and tryptophan metabolites). The polyhalogenated aromatic hydrocarbons are persistent environmental pollutants, and their lipophilicity and subsequent bioaccumulation through the food chain results in chronic lifetime low-level human exposure. In rodent studies, these persistent AHR ligands have been shown to induce a wide variety of biological and pathological effects including alteration in expression of specific AHR-regulated gene subsets, altered cell growth, endocrine disruption and cancer. Alterations in expression of these specific regulated genes occur via a mechanism that involves a high affinity interaction of dioxins and related ligands with the AHR, a basic helix-loop-helix protein that functions as a ligand-activated transcription factor. While the relationship between exposure and observed health effects has been well established in rodent models, considerable scientific controversy exists in the assessment of human health risk posed by the persistent daily low-level exposure to these potent environmental contaminants. This is due to the fact that the mechanisms relating activation of the AHR by either persistent or non-persistent ligands to the subsequent development of adverse health effects have not been established. Specifically our research is focussed in three areas (1) Determination of the mechanism of carcinogenicity of TCDD in rodents (2) Identification of cell specific and species differences in the AHR-dependent transcriptional response network between humans and rodents (3) Assessment of the suitability of relative potency factors for the prediction of cancer risk posed by TCDD and structurally related persistent polyhalogenated aromatic hydrocarbons. Specific Aim 1. Mechanism of hepatocarcinogenicity of TCDD : The induction of liver tumors by TCDD in the female rat has consistently been used by regulatory agencies in the establishment of appropriate guidelines for human exposure to dioxin. One of the most well characterized responses to TCDD is the induction of cytochromes P450 such as CYP1B1, CYP1A1 and CYP1A2. While TCDD has been shown to be a potent tumor promoter in the rat liver, the mechanism of tumor induction by TCDD and relationship to induce gene expression in these rodent models is not known. It is hypothesized that the persistent TCDD mediated activation of the AHR and induction of the cytochromes P450 CYP1 and associated estradiol hydroxylase activities results in an estrogen-dependent induction of oxidative stress that leads to indirect genotoxicity and altered hepatocyte growth response. Specific Aim 2. Cell specific and species differences in AHR-dependent transcriptional response: Human exposure to dioxins has been associated with increased risk of human lung disease and cancer. In rodents, TCDD exposure induces lung tumors in rats and mice and recent studies have shown that in rats, TCDD induces metaplasia and hyperplasia in the alveolar/bronchiolar epithelium. The specific aim of this project is to determine the species concordance of transcriptional response to AHR ligands in lung epithelia. Specific Aim 3 . Relative potency of toxicity of TCDD and structurally related persistent polyhalogenated aromatic hydrocarbons; When considering the risk of human exposure to dioxins, it is well established that exposure is as complex mixtures of polyhalogenated aromatic hydrocarbons rather than to a single specific congener. To assess the risk of these mixtures the Toxic Equivalency Factor (TEF) approach has been developed to characterize the toxicity of mixtures of those persistent compounds that bind to the AHR and possess "dioxin-like" activity. The specific aims are two-fold; initially we shall determine individual relative potencies for dioxin-like compounds and mixtures of these compounds, and the impact of co-exposure of non-dioxin-like PCBs on the potency for dioxin-like PCBs. Secondly, we shall determine the additivity of relative potencies for dioxins and their ability to predict the potency of mixtures of dioxins to induce gene expression, hormonal dysregulation, toxicity, and cancer. Research accomplishments. Demonstrated that the induction of 8-oxo-deoxyguanosine adducts by TCDD as a marker of TCDD induced indirect oxidative DNA damage is female-specific, estrogen dependent, and requires persistent chronic exposure. These data support the hypothesis that TCDD may be indirectly genotoxic via a chronic induction of oxidative stress as a result of AHR receptor-mediated induction of cytochrome P450 isozymes and subsequent production of redox active estrogen metabolites. Defined the TCDD-responsiveness of a recently established in vitro cell model of normal human lung Clara cells, the target for TCDD action in both human and rodent lung in vivo. Used toxicogenomic and quantitative expression analyses to investigate the comparative dose-dependent transcriptional network of TCDD action in "normal" vs malignant human peripheral lung epithelial cell lines. Identified pathways involved in cytokine and growth factor-signalling and cell adhesion as specific targets altered by TCDD that may be involved in TCDD induced neoplasia in the human lung. Integrated quantitative gene expression data into a current state-of-the-art physiologically based pharmacokinetic and mechanistic model for TCDD action. Used this refined model as a replacement for the use of simple default pharmacokinetic methods for the benchmark-dose estimation of body burdens of TCDD associated with a 1% excess risk for non-cancer endpoints. These analyses indicated that the use of default methods may lead to underestimation of the non-cancer risk posed by exposure to TCDD. Dose response modeling of CYP1 enzyme activity from interim necropsies was conducted. Analysis of independent model fits indicated that relative potency factors based on independent ED50 determinations were inclusive of the current TEF values for the respective chemicals tested and that the relative potency of the TEF mixture, designed using the current TEFs was inclusive of the expected value of 1.0. A rigorous statistical analysis of simultaneous model fits of the induced enzyme activity data indicated the relative potencies for the three congeners for CYP1A2 differed significantly from published TEF values. However, the relative potency of the mixtures of these congeners was not significantly different from additivity, when the new estimated potencies were applied.